1. Field of the Invention
The invention relates to the field of lithography for microelectronics and more particularly to high resolution lithography for use in forming dense, small, accurately dimensioned features.
2. Prior Art
Microelectronic lithography utilizing photoresists which are sensitive to visible or ultraviolet light have been in use for a number of years. These photoresists and associated exposure techniques have been a mainstay of the microelectronic arts and have been utilized for the production of countless microelectronic devices. However, as the state of the art in microelectronics has progressed, devices requiring smaller and smaller dimensions have been developed. These increasingly smaller devices have required constantly improving lithographic resolution in order to achieve satisfactory pattern definition and device yields. With currently available visible and ultraviolet light dependent exposure techniques, the minimum linear dimension of a feature of a microelectronic structure which can be accurately replicated is about 1 micron. Greater accuracy is not obtainable utilizing visible and ultraviolet actinic radiation because this actinic radiation has a sufficiently long wavelength that diffraction limits the achievable accuracy and resolution.
The adverse effects produced by diffraction can be minimized by utilization of conformable contact masks with visible and ultraviolet exposing radiation. However, contact of the mask with a microelectronic structure during manufacturing tends to introduce defects in the finished device by abrasion and contamination. Further, the resolution utilizing this technique is limited by the presence of dust particles which themselves act as individual masks and produce local separations between the mask and the substrate thereby degrading resolution.
In order to improve lithographic resolution over that obtained with ultraviolet and visible light systems, exposure techniques utilizing electron beams have been developed. Such systems provide excellent resolution but are not efficient large scale production techniques because of the high cost of exposure equipment and because present system require that the electron beam essentially draw the desired exposure pattern on the resist layer. The time required for the exposure of a pattern on an individual resist coating could be substantially reduced if electron projection lithography could be utilized. However, to date election projection lithgraphy has not been successful, inter alia, because of distortion problems.
In another attempt to improve on visible and ultraviolet light systems, X-ray lithography has been developed. This technique provides for exposure of entire wafers simultaneously. Unfortunately, special equipment is required because of the difficulty of generating well collimated X-rays of high intensity, and this complicates the overall system. Further, improved alignment techniques are needed in order to take full advantage of the benefits which may be provided by X-ray exposure.
Also as discussed in detail in an article entitled "Replication of 0.1.mu. Geometries with X-ray Lithography" by Feder et al., Journal of Vacuum Science Technology, Vol. 12, No. 6, November/December 1975, pages 1332-1335, secondary electron generation upon absorption of X-rays inherently limits the resolution obtainable with X-ray lithography. The degree to which secondary electron emission limits the resolution of X-ray lithography depends on the range of the secondary electrons in the photoresist. To minimize the resolution problems of X-ray lithography, Feder et al recommend the use of carbon K.alpha. low energy X-rays which generate secondary electrons having a minimum range in the resist.